Metallic and non-metallic substrates can be coated by reactive or non-reactive evaporation using conventional processes and apparatuses. Many useful engineering materials are routinely created by depositing thick and thin film layers onto surfaces using physical vapor deposition (PVD). The deposited layers vary in thickness from a few monolayers up to several millimeters. While many techniques are capable of creating layers of varying thickness, business economics in numerous market segments dictate that the most successful techniques will be able to create layers quickly and efficiently while also generating the precise atomic scale structures that bestow the engineering properties needed for the application. To create layers quickly, a process must be able to generate large amounts of vapor rapidly. To create layers efficiently, a process must be able to transport and deposit the majority of the vapor to specific desired locations, and mediate their assembly on the condensing surface to create structures of technological value.
Several methods can be used to organize atomic assembly to create a desired structure. For example, the substrate temperature, the deposition rate and the angle of incidence of the flux with the substrate where deposition occurs all affect the assembly process and therefore the resulting structure.
The capability of providing desired rapid, efficient, directed energy techniques, such as for thick and thin film coating applications, have continually alluded conventional practices. For some applications, high vapor atom energy (>20 eV) is needed to induce selective sputtering. For example to control grain texture by the selective removal of some crystal orientations. In other applications, medium energy (10-20 eV) is needed to densify the film and control its grain size and residual stress. In other cases (particular the growth of multilayers) modulated/pulsed low energy (<10 eV) deposition is used to grow each new layer. This low energy technique enables surfaces to be flattened without causing intermixing of the interfaces. Assisting ions with similar atomic masses to deposited species and with energies in the same three regimes can also be used to augment the deposition.
Speed
While conventional e-beam processing can be performed at very high rates, the vapor from a conventional e-beam source rapidly disperses as it moves away from its point of origin. As a result, the vapor from a conventional e-beam source leaves the feed stock with a density distribution often described by a cosn θ (θ: angle to normal axis) function where n ranges from 2 to 5. This diffuse distribution leads to coating nonuniformity for large area arrays and extremely low deposition efficiencies when coating, for example, small or curved surfaces such as 100 μm diameter continuous fibers with metal (to make composites) or 3 cm long aircraft engine turbine blades with ceramic (to make thermal barrier coatings). When a gas is introduced into the process chamber for reactive evaporation, the focus of the evaporant may become even worse (cos1 θ) as the result of collisions between the vapor stream atoms and background gas atoms in the chamber.
Efficiency
U.S. Pat. No. 5,534,314 to Wadley et al, of which is incorporated by reference herein, shows the efficiency of deposition in electron beam systems is enhanced by capturing vapor in a carrier gas stream as soon as it leaves the evaporator source so that it can be directed to a substrate as a focused beam for high rate, efficient deposition.
Energy
While the above described technologies combine deposition speed and efficiency, extensive study has demonstrated that they lack some key ingredient that allows them to form high quality layers possessing a wide range of easily selected microstructures—from porous, columnar to fully dense polycrystalline. They have a limited ability to combine efficient, high rate deposition with precisely selected deposition energies.
To generate plasma activation in a physical vapor deposition system, many technical variants based on low-pressure plasma discharges like glow-discharge plasma, thermionic cathode plasma, radio-frequency plasma and microwave plasma, with magnetic amplification, have been developed and applied for this purpose.
Despite the capabilities of some of the techniques just listed, their use has been confined to systems that do not operate in the 10−2 to 10 mbar regime of the present invention directed vapor deposition technique. Instead, because of their underlying inherent performance characteristics, their practical use is restricted to operation in a pressure range between 10−4 mbar and 0.1 mbar. Clearly, the important pressure regime used for rapid, efficient gas-guided vapor deposition is only partly covered by these conventional plasma processes.
Not only do these standard plasma techniques have difficulty operating in the higher pressure regime of directed vapor deposition but also they have a limited ability to ionize gas and vapor atoms in a system at any pressure. As long as the gas and vapor density in a system is relatively low, the listed techniques are capable of ionizing large percentages of the total atomic density. However, once the atomic density increases (e.g. in the high rate, elevated pressure regime of electron beam directed vapor deposition), the plasma density that can be created with these discharges is too small to achieve any effective improvement of the density in the deposited layers (See S. Schiller, H. Morgner, N. Schiller and S. Straach, High Rate Coating of Plastic Films and Plastic Sheets with Clear Oxide Layers, Paper presented on 1997 Joint International Meeting ECS/ISE in Paris, Aug. 31-Sep. 5, 1997, paper published in Metallized Plastics 5&6: Fundemental and Applied Aspects, p. 75-84; of which is hereby incorporated by reference in its entirety).
Although techniques like glow-discharge plasma, thermionic cathode plasma, radio-frequency plasma and microwave plasma, with magnetic amplification, are unable to create high plasma densities, it is known that a very high plasma density can be achieved by low voltage arc discharges (e.g. hollow cathode arc discharges). In the most commonly used setup of these discharges, the directed share of the plasma electrons (the beam electrons) is guided into the evaporator crucible, which is configured to act as an anode. For instance, as shown in U.S. Pat. No. 3,562,141 to Morley, of which is hereby incorporated by reference in its entirety, evaporation occurs due to absorption of kinetic energy of the low energy beam, and at the same time plasma ionization occurs in the system. However, this method cannot be transferred to gas stream guided vapor deposition. However, to penetrate the high particle density of a gas and vapor directed vapor deposition system, it is necessary to apply very high acceleration potential to the electrons (e.g. 60-70 kV for electron acceleration in Wadley et al. system), and therefore precludes use of this type of low voltage arc configuration in directed vapor deposition systems.
Another gas jet film deposition system uses a noble gas plasma jet formed by a nozzle with a thermionic electron emitter inside. The gas plasma jet is targeted on the evaporant or evaporation crucible acting as anode. The vaporized particles will be entrained in the gas plasma jet which continues its flow to a substrate (See U.S. Pat. No. 5,571,332 to Halpern; of which is hereby incorporated by reference herein).
In this Halpern configuration the plasma discharge is extended only in the noble gas atmosphere between the thermionic cathode and the evaporant as anode. So the vapor particles are not included in the discharge area. Therefore the jet after passing the anode consists mainly of neutral and ionized gas particles and neutral vapor particles. Ionization of vapor particles can happen only in a low degree by charge transfer.
This Halpern method is not suited for high rate deposition by two reasons. Firstly, the energy available for evaporation is mainly supplied by the anode fall of the plasma discharge. This energy is insufficient for high rate evaporation. Secondly, the density of plasma by thermionic electron emission is comparably much lower then the plasma by hollow cathode arc discharge and the vapor is not exposed directly to the fast electrons accelerated in the cathode fall. Therefore, a dense vapor plasma is necessary for high degree of activation at high rates can not be generated by Halpern.
In other experimental setups, low voltage arc sources, in particular hollow cathode arc sources, are used in an independent arrangement exclusively for activation of the vapor near a large-surface substrate (See German Patents DE 42 35 199 C1, DE 196 12 344 A1, DE 38 14 652 C2; all of which are hereby incorporated by reference in their entirety). Still, these arrangements are not well suited for plasma activation of a concentrated vapor beam with very high particle density flowing with high velocity. The patents cited above aim at plasma activation of an extended substrate surface with high degree of uniformity. Therefore the concentrated, unextended plasma from the cathode of the hollow cathode arc source will be spread utilizing for activation the much fewer dense plasma of the positive column in a certain distance from the cathode. The spread plasma does not match effectively with the localized gas and vapor stream. The large distance between the cathode orifice and the vapor will result in thermalisation of fast electrons in a high pressure directed vacuum deposition system before reaching the vapor particles which are concentrated in the carrier gas stream. Effective plasma activation of the jet will not take place in the above-listed German references.
Furthermore, the arrangements of the above-listed German references, use magnetic fields for the guidance of the plasma. Such magnetic fields can not be strongly localized resulting in an unacceptable interference of the high-energy e-beam impacting its ability to function properly for evaporation negatively.
For directed vapor deposition systems, a better plasma source configuration appears to be that described in DE 19841012 (herein after “DE'012”); of which is hereby incorporated by reference in its entirety. Here a hollow cathode arc plasma source, with a ring anode nearby the cathode orifice, is used in combination with a magnetic field and a ring-shaped anode to enclose the focused gas and vapor stream. The hollow cathode is arranged such that it is a considerable distance from the vapor beam, and electromagnetic field-lines are allowed to draw the directed electrons from the hollow cathode arc plasma source partly to the substrate and partly to the ring-shaped anode.
Still, this DE'012 configuration has several substantial drawbacks. First, the pathway for the electrons of the directed low voltage electron beam, from the orifice of the hollow cathode tube to the vapor beam, is too long. This results in a drastic energy loss for the low voltage beam electrons in the isotropic residual gas surrounding the vapor beam. The energy loss occurs before the beam has even entered the region of the gas stream guided vapor due to the scattering processes in the dense residual gas. This scattering gains importance for vacuum pressures above 0.05 mbar where elastic and inelastic scattering of electrons in the residual gas results in a decreased degree of plasma activation in the vapor and a corresponding decrease in the self bias voltage on the substrate surface. It is not unreasonable to experience a drop in the self bias voltage on an insulated substrate surface from typical levels of 15-20V for hollow cathode arc plasma activated processes in the lower pressure range to just a few volts, purely because of energy losses in the electrons. The self bias voltage is a critical factor, which determines the energy of condensing vapor ions and consequently the effect of plasma activation on layer properties. For example, for high deposition rates above 10 μm per minute a low degree of plasma activation will result in only a weak improvement of layer properties.
Second, this DE'012 configuration needs external magnetic field for plasma guidance. Due to the small distances in DVD configuration the magnetic fields from the plasma guidance and the strong circular magnetic field surrounding the power supply cables and the plasma discharge resulting from the high discharge current of the hollow cathode arc discharge, typically in the range of about one hundred amperes, leads to relatively high magnetic field strengths could negatively impact the ability of the high energy e-beam to function properly for evaporation. The execution example in the patent DE 19841012 (of which is incorporated by reference in its entirety) shows a shield between the plasma source and the evaporation e-beam's pathway for decoupling the evaporation e-beam from magnetic disturbance. However, the shielding must be kept outside the vapor channel. This means that shielding is not possible in the vicinity of the vapor source crucible. This restrictions makes it difficult to ensure that the e-beam pathway close to the crucible is not influenced by the magnetic field of the plasma discharge in the system configuration described in DE'012. Nonsystematic deviations from constant plasma conditions can often occur in hollow cathode arc discharges, leading to sudden, undesirable shifts in the position of the evaporation spot in the crucible. Such position shift of the evaporation spot makes the evaporation unstable, especially from crucibles with small diameter. There is therefore a need in the art wherein hollow cathode arc discharges can be advantageously incorporated into the directed vapor deposition process.
Finally, while Wadley et al. provides that an ion assisted directed vapor deposition is conceivable, no teaching about how such a process might be realized is provided, and there may have been an assumption that the high voltage electron beam (60 kV) would be used for source evaporation that might interact with gas and vapor in the process chamber to generate large percentages of ionized gas and vapor for deposition. However, high voltage electrons are not well suited to the creation of high ionization percentages due to the dramatic decreasing of the ionization cross section with high electron energy. Instead, a low energy beam as described in the present invention is much better suited to creation of large numbers of ionized species in a physical vapor deposition system.
Steep increase of ionization cross section for electron energy exceeding the ionization threshold energy with growing energy is well known. The hollow cathode arc discharge source is characterized by emitting of directed electrons, also called as low voltage electron beam, with an enhanced energy compared to the isotropic electrons in the plasma. The graph in FIG. 5 provides two curves. The thin plot shows a typical electron energy distribution of a hollow discharge arc plasma, which can be found typically away from the cathode in a certain distance. This curve can be described as Maxwellian distribution. The thick squared curve represents the typical distribution close to the cathode of a hollow cathode arc discharge consisting of a peak from the isotropic electrons on the left and the portion of the directed electrons on the right side. The energy distribution of the directed electrons with a mean energy of about 12 eV exceeds to a large portion the ionization threshold of vapor particle and reactive gases enabling very effective ionization of vapor and gas
Other prior art systems and method are captured in U.S. Pat. No. 5,635,087 to Schiller et al. and U.S. Pat. No. 4,941,430 to Watanabe et al., and are hereby incorporated by reference.
To expand the viable applicability of plasma activated electron beam directed vapor deposition technology, processes and systems are needed which are inter alia:    1. Capable of rapidly varying the energy of depositing atoms across the entire energy range from 0.05 eV to 300 eV.    2. Able to introduce plasma activation electrons from a directed low voltage electron beam directly into a focused gas and vapor stream destined for deposition.    3. Free from the disturbing influences of magnetic fields generated by the high current of the plasma activation unit.    4. Controllable (well defined energy modulation in the plasma activation unit and the substrate bias system).    5. Efficient (ionizing as much of the gas and vapor stream as possible).    6. Flexible (allowing many different gas and vapor types to be activated using a one or more plasma activation units.    7. Not operator intensive (i.e., continuous, automated, and reliable).